Phytoplankton dynamics in relation to hydrography, nutrients and zooplankton at the onset of sea ice formation in the eastern Weddell Sea (Antarctica)

Abstract

The quantitative and qualitative distribution of phytoplankton was investigated along five North- South transects in the eastern Weddell Sea during the transition from late autumn to winter. Relationships with the regional hydrography, progressing sea ice cov- erage, nutrient distribution and zooplankton are dis- cussed and compared with data from other seasons. To the north of the Antarctic Slope Front (ASF) a remnant temperature minimum layer was found above the pri- mary pycnocline throughout summer. Surface waters had not entirely acquired typical winter characteristics. While temperature was already in the winter range, this was not the case for salinity. Highest biomass of phy- toplankton, with the exception of the first transect, was found in the region adjoining the ASF to the north. Absolute chlorophyll a (Chl a) concentrations dropped from 0.35 to 0.19 l gl � 1 . Nutrient pools exhibited a replenishing tendency. Ammonium concentrations were high (0.75-2 lmol l � 1 ), indicating extensive heterotro- phic activity. The phytoplankton in the ASF region was dominated by nanoflagellates, particularly Phaeocystis spp.. North of the ASF the abundance of diatoms in- creased, with Fragilariopsis spp., F. cylindrus and Tha- lassiosira spp. dominating. Community structure varied both due to hydrographical conditions and the advanc- ing ice edge. The phytoplankton assemblage formed during late autumn were very similar to the ones found in early spring. A POC/PON ratio close to Redfield, decreasing POC concentration and a high phaeophytin/ Chl a ratio, as well as a high abundance of mesozoo- plankton indicated that a strong grazing pressure was exerted on the phytoplankton community. A compari- son between primary production (PP) in the water col- umn and the sea ice showed a shift of the major portion of PP into the ice during the period of investigation.